Homoserine and threonine pools of borrelidin resistant Saccharomyces cerevisiae mutants with an altered aspartokinase

Functional analysis of feedback inhibition-insensitive aspartate kinase identified in a threonine-accumulating mutant of Saccharomyces cerevisiae

Humans and mammals need to ingest essential amino acids (EAAs) for protein synthesis. In addition to their importance as nutrients, EAAs are involved in brain homeostasis. However, elderly people are unable to efficiently consume EAAs from their daily diet due to reduced appetite and variations in the contents of EAAs in foods. On the other hand, strains of the yeast Saccharomyces cerevisiae that accumulate EAAs would enable elderly people to intakegest adequate amounts of EAAs and thus might slow down the neurodegenerative process, contributing to the extension of their healthy lifespan. In this study, we isolated a mutant (strain HNV-5) that accumulates threonine, an EAA, derived from a diploid laboratory yeast by conventional mutagenesis. Strain HNV-5 carries a novel mutation in the HOM3 gene encoding the Ala462Thr variant of aspartate kinase (AK). Enzymatic analysis revealed that the Ala462Thr substitution significantly decreased the sensitivity of AK activity to threonine feedback inhibition even in the presence of 50 mM threonine. Interestingly, Ala462Thr substitution did not affect the catalytic ability of Hom3, in contrast to previously reported amino acid substitutions that resulted in reduced sensitivity to threonine feedback inhibition. Furthermore, yeast cells expressing the Ala462Thr variant showed an approximately threefold increase in intracellular threonine content compared to that of the wild-type Hom3. These findings will be useful for the development of threonine-accumulating yeast strains that may improve the quality of life in elderly people.IMPORTANCEFor humans and mammals, essential amino acids (EAAs) play an important role in maintaining brain function. Therefore, increasing the intake of EAAs by using strains of the yeast Saccharomyces cerevisiae that accumulate EAAs may inhibit neurodegeneration in elderly people and thus contribute to extending healthy lifespan and improving their quality of life. Threonine, an EAA, is synthesized from aspartate. Aspartate kinase (AK) catalyzes the first step in threonine biosynthesis and is subject to allosteric regulation by threonine. Here, we isolated a threonine-accumulating mutant of S. cerevisiae by conventional mutagenesis and identified a mutant gene encoding a novel variant of AK. In contrast to previously isolated variants, the Hom3 variant exhibited AK activity that was insensitive to feedback inhibition by threonine but retained its catalytic ability. This resulted in increased production of threonine in yeast. These findings open up the possibility for the rational design of AK to increase threonine productivity in yeast.

Effect of gene amplification on threonine production by yeast

In this work, we have studied the effect of amplifying different alleles involved in the threonine biosynthesis on the amino acid production by Saccharomyces cerevisiae. The genes used were wild-type HOM3, HOM2, HOM6, THR1, and THR4, and two mutant alleles of HOM3 (namely HOM3-R2 and HOM3-R6), that code for feedback-insensitive aspartate kinases. The results show that only the amplification of the HOM3 alleles leads to threonine and, in some instances, to homoserine overproduction. In terms of the regulation of the pathway, the data indicate that the main control is exerted by inhibition of the aspartate kinase and that, probably, a second and less important regulation takes place at the level of the homoserine kinase, the THR1 gene product. However, amplification of THR1 in two related Hom3-R2 strains does not increase the amount of threonine but, in one of them, it does induce accumulation of more homoserine. This result probably reflects differences between these strains in some undetermined genetic factor/s related with threonine metabolism. In general, the data indicate that the common laboratory yeast strains are genetically rather heterogeneous and, thus, extrapolation of conclusions must be done carefully. (c) 1996 John Wiley & Sons, Inc.

A new mutation in the yeast aspartate kinase induces threonine accumulation in a temperature-regulated way

In Saccharomyces cerevisiae, aspartate kinase (the HOM3 product) regulates the metabolic flux through the threonine biosynthetic pathway through feedback inhibition by the end product. In order to obtain a strain able to produce threonine in a controlled way, we have isolated a mutant allele (HOM3-ts31d) that gives rise to a deregulated aspartate kinase. This allele has been isolated as an extragenic suppressor of ilv1, which confers an Ilv+ phenotype at 37 degrees C but not at 22 degrees C. We have stated that at high temperature the mutant aspartate kinase is slightly more deregulated and shows a higher specific activity, inducing threonine accumulation. The HOM3-ts31d allele carries a mutation that leads to a Ser399 --> Phe substitution in the postulated regulatory region of the enzyme. We have detected other changes in the nucleotide sequence but they are also present in the parental strain, reflecting the genetic differences between different wild-type strains. A sequence comparison among all the reported mutant aspartate kinases suggests that not all residues involved in regulation of the activity are clustered in the so-called regulatory domain, as is the case of that mutated in AK-R7, another deregulated aspartate kinase obtained with the same strategy of ilv1 suppression.

Characterization of the aspartate kinase from Saccharomyces cerevisiae and of its interaction with threonine

Aspartate kinase (AK) from Saccharomyces cerevisiae has been characterized to elucidate its quaternary structure and the effect of the allosteric inhibitor threonine on the enzyme conformation. The homogeneously purified enzyme was inhibited by threonine (K(i) 1.4 mM) and was found to bind this compound (K(d) 0.97 mM) in a hyperbolic manner. Gel filtration and native gel electrophoresis indicated that yeast AK is a homohexamer of 346 kDa composed by 58 kDa subunits. Threonine caused a decrease in the apparent molecular mass of AK as evidenced by size-exclusion chromatography (from 345 to 280 kDa) and blue native gel electrophoresis (from 346 to 297 kDa); no other molecular species were detected. This shift in the hydrodynamic size was threonine-specific and was reversed by rechromatography in the absence of threonine. No change in the apparent molecular mass was induced by threonine in an AK mutant insensitive to inhibition by this amino acid, which was observed to be unable to bind threonine. These results indicate that the allosteric transition elicited by binding of threonine to yeast AK involves a large conformational change of the protein that isomerizes from a relaxed active conformation to a more compact inactive one of smaller molecular dimensions.

Enrichment of threonine content in Saccharomyces cerevisiae by pathway engineering

In a previous work, we have investigated the effect of amplifying individually the genes of the threonine biosynthetic pathway on threonine accumulation by yeast. Here, we present the results of the simultaneous amplification of these genes in strains with different genetic backgrounds. These strains carry a mutant HOM3-R2 allele (coding for a feedback-insensitive aspartate kinase), and/or a mutant cha1 allele that makes it defective in threonine degradation by the catabolic L-serine (L-threonine) deaminase. The results show that the amplification of the clustered genes affects threonine and homoserine accumulation only when it includes the HOM3 gene, or when combined with a HOM3-R2 mutation. Similarly, the cha1 mutation is only effective when a certain amount of threonine is reached. Threonine overproduction affects other cellular functions such as the accumulation of other amino acids, the cell growth and metabolite excretion, probably reflecting a redirection of the carbon flux in the central metabolism.

Mutations that cause threonine sensitivity identify catalytic and regulatory regions of the aspartate kinase of Saccharomyces cerevisiae

The HOM3 gene of Saccharomyces cerevisiae encodes aspartate kinase, which catalyses the first step in the branched pathway leading to the synthesis of threonine and methionine from aspartate. Regulation of the carbon flow into this pathway takes place mainly by feedback inhibition of this enzyme by threonine. We have isolated and characterized three HOM3 mutants that show growth inhibition by threonine due to a severe, threonine-induced reduction of the carbon flow into the aspartate pathway, leading to methionine limitation. One of the mutants has an aspartate kinase which is 30-fold more strongly inhibited by threonine than the wild-type enzyme. The predicted amino acid substitution in this mutant, A406T, is located in a region associated with the modulation of the enzymatic activity. The other two mutants carry an aspartate kinase with reduced affinity for its substrates, aspartate and ATP. The corresponding amino acid substitutions, K26I and G25D, affect residues located in the vicinity of a highly conserved lysine-phenylalanine-glycine-glycine (KFGG) stretch present in the N-terminal part of the aspartate kinase, to which no function has so far been assigned. We suggest that this region is involved in substrate binding. Mutagenesis of a HOM3 region centred in the KFGG-coding triplets generated alleles that determine threonine sensitivity or auxotrophy for threonine and methionine, but not a phenotype associated with a feedback-resistant aspartate kinase, indicating that this region is not involved in the allosteric response of the enzyme.

Threonine overproduction in yeast strains carrying the HOM3-R2 mutant allele under the control of different inducible promoters

The HOM3 gene of Saccharomyces cerevisiae codes for aspartate kinase, which plays a crucial role in the regulation of the metabolic flux that leads to threonine biosynthesis. With the aim of obtaining yeast strains able to overproduce threonine in a controlled way, we have placed the HOM3-R2 mutant allele, which causes expression of a feedback-insensitive enzyme, under the control of four distinctive regulatable yeast promoters, namely, PGAL1, PCHA1, PCYC1-HSE2, and PGPH1. The amino acid contents of strains bearing the different constructs were analyzed both under repression and induction conditions. Although some differences in overall threonine production were found, a maximum of around 400 nmol/mg (dry weight) was observed. Other factors, such as excretion to the medium and activity of the catabolic threonine/serine deaminase, also affect threonine accumulation. Thus, improvement of threonine productivity by yeast cells would probably require manipulation of these and other factors.

Isolation of a mutant allele that deregulates the threonine biosynthesis in Saccharomyces cerevisiae

We have cloned the yeast allele HOM3-R2, that codes for a mutant aspartate kinase which is insensitive to feedback inhibition by threonine, by gap-repair. A strain carrying this allele in a multicopy plasmid, or integrated into the genome, accumulates 14-times and 8-times more threonine than the wild-type, respectively. The sequence of the mutant allele differs from that of the wild-type in a single base pair change, namely a G by an A, at position 1355 in the open reading frame. The fact that the presence of this mutant allele in a cell induces threonine overproduction points to aspartate kinase as the key enzyme in the regulation of threonine biosynthesis in yeast.

Editing of errors in selection of amino acids for protein synthesis

All living cells must conduct protein synthesis with a high degree of accuracy maintained in the transmission and flow of information from gene to finished protein product. One crucial "quality control" point in maintaining a high level of accuracy is the selectivity by which aminoacyl-tRNA synthetases furnish correctly activated amino acids, attached to tRNA species, as the building blocks for growing protein chains. During selection of amino acids, synthetases very often have to distinguish the cognate substrate from a homolog having just one fewer methyl group in its structure. The binding energy of a methyl group is estimated to contribute only a factor of 100 to the specificity of binding, yet synthetases distinguish such closely related amino acids with a discrimination factor of 10,000 to 100,000. Examples of this include methionine versus homocysteine, isoleucine versus valine, alanine versus glycine, and threonine versus serine. Many investigators have demonstrated in vitro the ability of certain aminoacyl-tRNA synthetases to edit, that is, correct or prevent incorrect attachment of amino acids to tRNA molecules. Several major editing pathways are now established from in vitro data. Further, at least some aminoacyl-tRNA synthetases have recently been shown to carry out the editing function in vivo. Editing has been demonstrated to occur in both Escherichia coli and Saccharomyces cerevisiae. Significant energy is expended by the cell for editing of misactivated amino acids, which can be reflected in the growth rate. Because of this, cellular levels of aminoacyl-tRNA synthetases, as well as amino acid biosynthetic pathways which yield competing substrates for protein synthesis, must be carefully regulated to prevent excessive editing. High-level expression of recombinant proteins imposes a strain on the biosynthetic capacity of the cell which frequently results in misincorporation of abnormal or wrong amino acids owing in part to limited editing by synthetases. Unbalanced amino acid pools associated with some genetic disorders in humans may also lead to errors in tRNA aminoacylation. The availability of X-ray crystallographic structures of some synthetases, combined with site-directed mutagenesis, allows insights into molecular details of the extraordinary selectivity of synthetases, including the editing function.

Overproduction of threonine by Saccharomyces cerevisiae mutants resistant to hydroxynorvaline

In this work, we isolated and characterized mutants that overproduce threonine from Saccharomyces cerevisiae. The mutants were selected for resistance to the threonine analog alpha-amino-beta-hydroxynorvalerate (hydroxynorvaline), and, of these, the ones able to excrete threonine to the medium were chosen. The mutant strains produce between 15 and 30 times more threonine than the wild type does, and, to a lesser degree, they also accumulate isoleucine. Genetic and biochemical studies have revealed that the threonine overproduction is, in all cases studied, associated with the presence in the strain of a HOM3 allele coding for a mutant aspartate kinase that is totally or partially insensitive to feedback inhibition by threonine. This enzyme seems, therefore, to be crucial in the regulation of threonine biosynthesis in S. cerevisiae. The results obtained suggest that this strategy could be efficiently applied to the isolation of threonine-overproducing strains of yeasts other than S. cerevisiae, even those used industrially.

Four major transcriptional responses in the methionine/threonine biosynthetic pathway of Saccharomyces cerevisiae

Genes encoding enzymes in the threonine/methionine biosynthetic pathway were cloned and used to investigate their transcriptional response to signals known to affect gene expression on the basis of enzyme specific-activities. Four major responses were evident: strong repression by methionine of MET3, MET5 and MET14, as previously described for MET3, MET2 and MET25; weak repression by methionine of MET6; weak stimulation by methionine but no response to threonine was seen for THR1, HOM2 and HOM3; no response to any of the signals tested, for HOM6 and MES1. In a BOR3 mutant, THR1, HOM2 and HOM3 mRNA levels were increased slightly. The stimulation of transcription by methionine for HOM2, HOM3 and THR1 is mediated by the GCN4 gene product and hence these genes are under the general amino acid control. In addition to the strong repression by methionine, MET5 is also regulated by the general control.

Inhibition by different amino acids of the aspartate kinase and the homoserine kinase of the yeast Saccharomyces cerevisiae

In this paper, we describe a simple method to measure the yeast homoserine kinase and aspartate kinase activities, independently but in the same extract. With this method, we have determined some kinetic parameters for the physiological substrates of both enzymes, and investigated the inhibition exerted by different amino acids on these activities. Of all natural amino acids tested, only threonine inhibits effectively both enzymatic activities, although to a different degree. We did not find the reported inhibition by L-homoserine over the aspartate kinase. Altogether the data point to the aspartate kinase and to the threonine as the key factors in the regulation of this route.

Aspartate kinase and homoserine dehydrogenase of Candida utilis

Aspartate kinase and homoserine dehydrogenase activity were assayed in a dialyzed cell-free extract of Candida utilis. Aspartate kinase was partly inhibited by ATP-Mg and by Mg2+ alone. There appear to be two isoenzymes of aspartate kinase in the yeast, one heat-labile, the other relatively heat-stable. The first is subject to feedback inhibition by threonine, the other is threonine-resistant. Neither aspartate kinase nor homoserine dehydrogenase is the rate-limiting enzyme in methionine biosynthesis. Homoserine dehydrogenase measured in the forward direction showed an activity five times higher than aspartate kinase. No regulatory interaction could be demonstrated for this enzyme. No repression of aspartate kinase and homoserine dehydrogenase synthesis by threonine, methionine or both amino acids was observed.

Feedback-insensitive aspartate kinase isoenzymes in barley mutants resistant to lysine plus threonine

The regulatory properties of aspartate kinase (EC 2.7.2.4) and homoserine dehydrogenase (EC 1.1.1.3) in two barley (Hordeum vulgare L.) mutants resistant to growth inhibition by lysine plus threonine, Rothamsted (R) 3004 and R3202, were compared with those in the normal, sensitive parent line cv. Bomi. Three forms of aspartate kinase (AKI, AKII, AKIII) were chromatographically separated and were considered to represent at least three independently regulated isoenzymes. Aspartate kinase I was inhibited by threonine; AKII and AKIII by lysine or lysine plus S-adenosylmethionine. The characteristics of AKI were unchanged in the mutants. Aspartate kinase II and AKIII from Bomi were both inhibited by lysine and by lysine plus S-adenosylmethionine. Aspartate kinase II from mutant R3202 was altered in its properties such that it was insensitive to lysine or lysine plus S-adenosylmethionine; AKII from mutant R3004 did not differ in its properties from AKII of Bomi. The concentration of lysine required to give half maximal inhibition of AKIII from R3004 was ten times that required for AKIII of Bomi; AKIII from R3202 did not differ from that of Bomi in this regard. There was no change in the properties of homoserine dehydrogenase of the mutants as compared with that of Bomi. We conclude that the lt1 and lt2 loci code for structural genes for lysine- and lysine plus S-adenosylmethionine-sensitive aspartate kinase isoenzymes. The mutant genes Lt1b and Lt2 in R3202 and R3004 respectively code for feedback-desensitized isoenzymes. The presence of one of these is sufficient to allow the synthesis of methionine to overcome the growth inhibition by lysine plus threonine.